MFE 3004 Mechatronics I Measurement Systems Dr Conrad Pace Page 4.1 Introduction to Measurement Systems Role of Measurement Systems Detection receive an external stimulus (ex. Displacement) Selection measurement of one property of that stimulus (ex. direction of displacement, filtering our disturbances) Signal Management transform signal that represents the measured property in a form legible by the information processor (ex. amplification, linearisation, digital conversion) Communication communicate signal to the observer/ information processor Page 4.2
Introduction to Measurement Systems A sensor is a device capable of detecting a physical parameter Receives energy from the measured medium (stimulus) It produces a signal output which depends on the stimulus Will always extract energy from the measured medium Transducers are devices which convert a physical parameter into another (often being a conversion from a physical parameter into an electrical quantity). Not all transducers are sensors but all sensors are transducers Change in temperature Transducer (Thermistor) Change in Resistance The measurand is the quantity, property, or condition that is measured (i.e. sensed and converted into a usable electrical output), by a transducer Page 4.3 Transducer Classification Transducer classification measurand based or physical effect based. Classification by Measurand Displacement Velocity Acceleration Angle Angular Velocity Torque Pressure Flow rate Time Temperature Radiation Magnetic Flux, etc.. Classification by Physical Effect Variable Resistance Variable Capacitance Variable Inductance Deformation of elastic materials Seismic masses Piezo-electric effect Optical interference Electro-magnetic induction Hall effect Thermo-resistivity Thermo-expansion Thermo-electric (Seebeck) effect Photo-electric effect, etc.. Page 4.4
Measurand Sensor Output Relation The relation between the measurand and the sensor output is generally clearly identifiable and linear. Sensor Output O/P = Const x Measurand Measurand Page 4.5 Overview of Sensor Technologies Displacement Contact Type Sensors Analogue Type Resistance (Potentiometric, Strain Gauge) Capacitive Inductive Digital Type Encoders Page 4.6
Overview of Sensor Technologies Displacement Contact Type Sensors Cantilever beam V ref R (1-k) (k) R L V out Strain α deflection (for measuring small distances Strain Gauge Application Potentiometer Application Page 4.7 Overview of Sensor Technologies Other Strain Gauge Sensor Applications Note: rounded corners to avoid stress concentration Diaphragm Force Force Cavity Strain gauge FORCE Load cell PRESSURE Strain gauge Strain gauges Seismic mass Torque Strain gauges (mounted at 45 to measure shear strain) TORQUE Torque Support ACCELERATION Beam Page 4.8
Overview of Sensor Technologies Displacement Contact Type Sensors Core displacement x x g Secondary 1 Primary Secondary 2 l ε 1 ε 2 g Inductive Application x Capacitive Application Page 4.9 Overview of Sensor Technologies Displacement Contact Type Sensors LED Light Sensor Disc Digital Rotary Encoder Page 4.10
Overview of Sensor Technologies Displacement Non-Contact Type Sensors Transducer (primary sensing element) Signal Conditioning stage Triggering stage Amplification Stage Detect proximity of object to sensor Various technologies are used for proximity sensors including Capacitive and Inductive. Page 4.11 Overview of Sensor Technologies Force Sensors Measuring small displacements caused by the force. Strain Gauge Load Cells Piezo-Electric Load Cells Force Piezo- Electric Material + + + + + + + ---------- Surfaces become charged LOAD CELLS using Strain Gauges Piezo-Electric effect Piezo-Electric Application Page 4.12
Overview of Sensor Technologies Temperature Sensors Use of expansion/ contraction of solids or liquids Measurement of gas pressures Change in electrical resistance Thermoelectric e.m.f. Radiation measurement Page 4.13 Overview of Sensor Technologies Temperature Sensors Temperature Range Measuring Instrument Method of Measurement 0.65 K to 5 K Gas Thermometer Measurement of vapour pressures of He (helium) using specified equations. 3 K to 24.6 K Gas Thermometer Measurement from a constant volume gas thermometer 14 K to 303 K Platinum resistance thermometer (PRT) Specified reference function together with a deviation equation whose coefficients are determined in the calibration against the fixed points. 0.01 C 962 C to Platinum resistance thermometer (PRT) Specified reference function and a deviation equation 962 C and above Radiation pyrometer Defined by Planck s law taking the radiation emitted from the body. Page 4.14
Basic Measurement System Components Data storage/ playback Element Measured Medium Measurand Primary sensing Element Signal Conditioning Element Signal Processing Element Data Transmission Element Data Presentation Element Presented Data Observer Power supply Page 4.15 Basic Measurement System Components Example of a Measurement System Resistance Input Pillar Load Cell Strain Strain Gauge True Weight (Measurand) Primary Secondary Bridge Circuit mv Amplifier Circuit V Sensing Signal Conditioning Output Measured Weight Visual Display Unit Microprocessor (linearisation and error compensation) A/D Converter Data Presentation Signal Processing Page 4.16
Application Areas of Measurement Systems Monitoring of Processes and Operations Control of Processes and Operations Most common application in mechatronic products and processes. Experimental Engineering Analysis In solving engineering problems, two general methods are available: theoretical and experimental. Measurement systems are a fundamental component of experimental work. Page 4.17 Sensory Characteristics Measurand Characteristics Electrical Characteristics Mechanical Characteristics Performance Characteristics Page 4.18
Sensor Measurand Characteristics Sensor Type is often defined by the Measurand Sensors can be used to measure other parameters indirectly due to a known relation between the parameter of interest and the measurand. The Range of the sensor is given by the upper and lower limits of measurand values to which the sensor will respond to within specified performance tolerances The Span is the algebraic difference between the two limits of the range. Example : Force Sensor Range 10 to 50kN Span 40kN Page 4.19 Sensor Electrical Characteristics Measurand Transducer Z in Z out Output Power Supply Z L Load Sensor Output Types Analogue Resistance Change, Capacitance Change, Inductance Change, Voltage Change, Charge build-up, frequency output Digital discrete function of the measurand Page 4.20
Sensor Mechanical Characteristics Mechanical Characteristics define the Physical interface of the sensor Mode of mounting Sensor Orientation Environmental Conditions to which the sensor is exposed (ex. vibration and mechanical stress) Page 4.21 Sensor Performance Characteristics Classification of Performance Characteristics Static Dynamic Environmental Page 4.22
Sensor Performance Characteristics 100 Steady-state relation Pressure (kpa) 0 100 200 300 400 500 600 700 800 900 1000 5.0 Output (% FSO) 90 80 70 60 50 40 30 Sensor output given as a % of the Full Scale Output (%FSO) 4.5 4.0 3.5 3.0 2.5 2.0 1.5 Output (Volts dc) 20 1.0 10 0.5 0 0.0 0 10 20 30 40 50 60 70 80 90 100 Measurand (% Range) Page 4.23 Sensor Performance Characteristics 100 90 80 70 Steady-state relation Pressure (kpa) 0 100 200 300 400 500 600 700 800 900 1000 5.0 4.5 4.0 3.5 Output (% FSO) 60 50 40 30 20 3.0 2.5 Relationship is derived from 2.0 either 1.5 1.0 Output (Volts dc) (a) By calculation from a known theoretical response 10 (b) By calibration 0.5 0 0.0 0 10 20 30 40 50 60 70 80 90 100 Measurand (% Range) Page 4.24
Sensor Performance Characteristics 100 Steady-state relation Pressure (kpa) 100kPa 0 100 200 300 400 500 600 700 800 900 1000 5.0 90 Sensitivity = 80 70 Change in Output Change in Measurand 0.5V 4.5 4.0 3.5 Output (% FSO) 60 50 40 30 Sensitivity = 5mV/kPa 3.0 2.5 2.0 1.5 Output (Volts dc) 20 1.0 10 0.5 0 0.0 0 10 20 30 40 50 60 70 80 90 100 Measurand (% Range) Page 4.25 Sensor Performance Characteristics Resolution The smallest change in measurand value that can be detected. For an Ideal ANALOGUE Sensor an infinitesimally small change in measurand will result in an equivalent change in output In practice the smallest detectable change is limited due to various reasons (electrical noise, friction, inertia, etc..) Page 4.26
Sensor Performance Characteristics Resolution Analogue Sensors Limited by noise Digital Sensors Limited by the value of the least significant bit of the digital output signal Resolution is often defined as a percentage of the full-scale output (% FSO) Example - Angular velocity sensor minimum change in speed detected 2 rad/s Maximum measurable input 200 rad/s Resolution = 1.0 %FSO Page 4.27 Sensor Performance Characteristics Accuracy and Errors (uncertainty) Errors distort the expected relationship between the measurand and sensor output Error = Measured Value True Measurand Value The Accuracy of a sensor is the ability to give an indication equivalent to the true value of the measurand (it is a reflection of the maximum error to be expected from the sensory device). Page 4.28
Sensor Performance Characteristics Accuracy Dependent on the Errors to which the sensor is subjected Reflects the precision of calibration of the sensor Is stipulated as a %FSO Example : Temperature Sensor Range 0 to 200 C Sensor Max. Error of ± 10 C Sensor Accuracy = ± 5% FSO Page 4.29 Sensor Performance Characteristics Error Types REPEATABILITY The ability of the sensor to give the same output for repeated applications of the same input measurand value Repeatability = Maximum Minimum Values Given x 100% FSO Page 4.30
Sensor Performance Characteristics Error Types HYSTERESIS The maximum difference in sensor output for a specific measurand value when the value is approached, first with increasing and then with decreasing measurand. 100 90 Output (% FSO) 80 70 60 50 40 30 20 10 Decreasing Measurand Increasing Measurand Hysteresis Error 0 0 10 20 30 40 50 60 70 80 90 100 Measurand (% Range) Page 4.31 Sensor Performance Characteristics Error Types NON-LINEARITY Deviation from an idealised linear relationship Reasons for non-linearity Techniques/ Phenomena used for sensing the measurand Non-linear characteristics of certain parameters arising from manufacturing variations (ex. Diaphragm properties) Page 4.32
Sensor Performance Characteristics Error Types NON-LINEARITY Interpretations of Non-Linearity Errors Terminal non-linearity Independent non-linearity Measured Output Actual Ideal Measured Output Actual Line of best fit Maximum deviation gives the non linearity error Measurand (a) Terminal non-linearity Maximum deviation gives the non linearity error Measurand (b) Independent non-linearity Page 4.33 Sensor Performance Characteristics Error Types Offset and Gain Errors Offset Error (Zero Shift) Shift of the straight line relationship from the origin Gain Error (Sensitivity Shift) Change in the straight line slope Reasons for zero shift/ sensitivity shift Environmental conditions (temperature fluctuations) Wear in sensor components, etc.. Page 4.34
Sensor Performance Characteristics Error Types Offset and Gain Errors Measured Output Actual Ideal Measured Output Actual Ideal Gain error Offset error (a) Offset Error Measurand (b) Gain Error Measurand Page 4.35 Sensor Performance Characteristics Error Types Stability The ability to give the same output when subjected to a constant measurand input over a period of time. Commonly referred to as the sensor drift or creep. Page 4.36
Sensor Performance Characteristics Error Types Dead Band The measurand range for which there is no output Measured Output Actual Dead-band Measurand Page 4.37 Sensor Performance Characteristics Dynamic Performance Characteristics Define the transient behaviour of the sensor The importance of dynamic characteristics depends on the rapidity of the sensor response required (compared to the system under control) Typical dynamic responses of interest Step Response (step change in measurand) Frequency Response (sinusoidal frequency change in measurand) Page 4.38
Sensor Performance Characteristics Dynamic Performance Characteristics Frequency Response Amplitude Ratio = Sensor Output Magnitude/ Measurand Magnitude Phase Shift = Phase lag between Sensor output and Measurand Page 4.39 Sensor Performance Characteristics Dynamic Performance Characteristics Frequency Response Frequency range A Frequency range B Response Curve A Response Curve B Page 4.40
Sensor Performance Characteristics Dynamic Performance Characteristics Step Response Percent of output change 100 90 80 70 60 50 40 30 20 10 95% 63.2% Dead Time 5% Transient Response Time Constant Response Time Rise Time Steady-State Response 0 0 Application of Measurand Change Time Page 4.41 Sensor Performance Characteristics Dynamic Performance Characteristics Step Response 160 Percent of output change 140 120 100 80 60 Maximum Overshoot Steady state output value 40 20 0 0 Application of Measurand Change Page 4.42
Sensor Performance Characteristics Environment Characteristics Describe the environmental effects on the sensor performance Example Temperature Offset and Gain Errors (amongst the most common environmental effects on sensors) %FSO per C change in temperature Temperature Offset Error is often given as a Temperature Sensitivity change in output which is solely due to changes in temperature Page 4.43 Sensor Performance Characteristics Environment Characteristics Reducing Temperature Effects Controlling the sensor s ambient temperature Compensating for temperature effects Using dummy sensors (a dummy sensor is subjected only to temperature effects and not the measurand change) Active temperature measurement and compensation Using low temperature coefficient materials/ low power dissipation components Repeated calibration in smart sensors Page 4.44
Sensor Performance Characteristics Environment Characteristics Other environmental factors that can influence the sensor output Humidity Pressure Mechanical Stress/ Strain Vibrations Electro-magnetic interference Electrostatics Page 4.45 Designing Measurement Systems Measurement Systems form a principal component within the design concept of a Mechatronic System Establish Information Requirements What information is required to be gathered and managed by the system Design Measurement Systems Identify Appropriate Sensory Technology and Signal Processing, Manipulation and communication requirements Page 4.46
Designing Measurement Systems Consideration of the following factors The information required and the identification of the system physical parameters that must be measured in order to provide this information. The nature, quality and performance of the measurement in terms of parameters such as linearity, accuracy and resolution. A determination of the most inaccurate measurement that would be acceptable (required accuracy) The effect on the system performance of any drift in the measurement circuit (zero or sensitivity) Page 4.47 Designing Measurement Systems Consideration of the following factors (continued) The environmental conditions under which the sensors are expected to operate The cost targets to be met The nature and form of the information transfer required The reliability of the system The form of the interface to adjacent modules in the system. Page 4.48
Designing Measurement Systems When designing Measurement Systems care should be taken not to provide too much or too little information Too Much Information Added cost Added Processing burden Too Little Information Inadequate Accuracy Insufficient Update Rate Lack of Desired Performance Page 4.49